Sometimes objects need to be milled out of a borehole. One such occasion is fracturing where a series of barriers such as plugs are set to isolate zones for delivery of high pressure fluid. After the zones are treated the barriers are commonly milled out. In some wells tubular patches have been installed for a variety of reasons such as to cover perforations that no longer producing or to reinforce the tubular string to address a variety of issues in the wall of the tubular. These patches known also as clads can be applied after the plugs are in position that later need to be milled out. The presence of a clad reduces the drift dimension of the tubular requiring a reduced diameter mill so it can pass through. This reduction in diameter to clear an obstruction increases the milling time and can result in incomplete milling because the peripheral parts of the plug are not reached by a mill made smaller to clear the smallest drift dimension that will be encountered.
Fixed outer dimension blades on mills such as illustrated in U.S. Pat. No. 5,720,349 have limited utility in such applications. One approach to the problem in the past has been to use applied pressure to move a piston in the mill that axially shifts a camming member having multiple ramp surfaces with the result being that the blades are pushed out radially through a surrounding opening as a result of the camming action. A good example of such a design is U.S. Pat. No. 8,561,724. Other examples in the same vein are U.S. Pat. No. 6,615,933; US 2002/0070052 and US 2004/0222022. U.S. Pat. No. 4,357,122 shows blades inserted into sockets on a mandrel for an end mill. The blades are fixed once installed giving the mill a constant outer dimension.
The pressure operated mills with extendable blades are expensive to build and require an array of seals that need to function in a hostile environment. Seal failures can mean that the blades either fail to extend or only partially extend. Additionally these mills tend to have large outer dimensions for the mandrel assembly as there is typically an annular cavity around the mandrel that is accessed by a port from the mandrel passage so that an annular piston can be pushed to axially shift camming members for blade extension. The camming mechanism can easily jam as it depends on axial actuator movement that then forces a taper under a blade so that the blade movement is exclusively radial. While some of these designs feature a return spring to retract the cam axially out from under the blade that has been pushed out radially, the reality is that the nature of such movements creates a real risk for a jam to an extent that the return spring will not be powerful enough to retract the blades to allow mill removal.
The present invention avoids the cost and complexity of pressure actuated blades that are cammed to move in a radial direction with a simple and innovative design that relies on set down weight on the blades to move them along a dovetail so that they radially extend as they are pushed relatively axially with respect to the mandrel. The blades are secured for running in so that the mill outer periphery is at the smallest dimension. Setting down weight releases a retainer and energizes a return spring that is held compressed during milling. Picking up the mill allows the spring to reverse the blade movement with respect to the mandrel for retraction of the blades. In another aspect of the invention, at least one stabilizer can be extended after the mill passes through tubing with the blades retracted so that the mill is stabilized with the blades extended during the milling operation. These and other aspects of the present invention will become more apparent from a review of the description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be determined from the appended claims.